When performing on-line cooling of a hot rolled steel plate, it is difficult to uniformly cool upper and lower surfaces of the steel plate with the same cooling capability. Particularly, on the lower surface, after impingement of cooling water upon the steel plate, the cooling water is immediately moved away by the force of gravity from the steel plate. As such, no cooling beyond cooling only with impinging water jets can be accomplished, so that the cooling capability for the lower surface is lower than that for the upper surface of the steel plate. For this reason, conventionally, uniformity of cooling has been implemented by changing a volume of the cooling water that is applied to the upper and lower surfaces of the steel plate. However, depending on temperature, thickness, and the like factors of the steel plate, and temperature of the cooling water, an optimal volume of the cooling water on the upper and lower surfaces is different. This makes it difficult to implement uniform cooling therefore facilitating occurrence of cooling nonuniformity. As such, the steel plate after cooling can have problems of deformation, residual stress, and nonuniformity in properties, consequently leading to operational troubles and deterioration in production yield.
In order to solve these problems, various cooling apparatuses have been proposed, such as those for enhancing cooling capability for a lower surface of steel plate and those for uniformly cooling upper and lower surfaces of steel plate.
Japanese Examined Patent Publication No. 63-4604 discloses a cooling apparatus as shown in FIG. 1.
This cooling apparatus has a water tank 2 provided with a predetermined spacing on a lower surface side of a steel plate 1; a round-tubular cooling nozzle 3 vertically fixed to a bottom portion of the water tank 2; and a conduit 4 that is vertically installed in an upper portion of the cooling nozzle 3 and that has a cross section substantially similar to a cross section of the cooling nozzle 3 and larger than the cross section of the cooling nozzle 3. A top portion of the cooling nozzle 3 and a bottom portion of the conduit 4 are positioned below the water surface, and a top portion of the conduit 4 is exposed above the water surface.
The cooling nozzle 3 having the conduit 4 is called an induced laminar flow nozzle (for water cooling) 6. The publication describes that the lower surface of the steel plate 1 can be stably and uniformly cooled by the nozzle, and the cooling capability can be controlled in a wide range.
Japanese Unexamined Patent Application Publication No. 10-166023 discloses a cooling apparatus as shown in FIG. 2.
This cooling apparatus has cooling nozzles 3A installed on an upper surface side of a steel plate 1 and cooling nozzles 3B installed on a lower surface side of a steel plate 1 between individual sets of transfer rollers 7. The number of the cooling nozzles 3B on the lower surface side is larger than the number of the cooling nozzles 3A on the upper surface side. In addition, between the individual sets of the transfer rollers 7, the cooling nozzles 3A and 3B are disposed so that cooling starts synchronously for the upper and lower surfaces of the steel plate 1. The publication describes that the arrangement equalizes cooling capabilities for the upper and lower surfaces of the steel plate 1. The publication further describes that when the induced laminar flow nozzles of the type described above are used for the cooling nozzles 3B on the lower surface side of the steel plate, even more uniform cooling can be implemented for the upper and lower surfaces, occurrence of distortion is prevented, and in addition, nonuniformity in properties is reduced.
However, problems remain even in the case that the cooling apparatus described in Japanese Examined Patent Publication No. 63-4604 or Japanese Unexamined Patent Application Publication No. 10-166023 is used. In this case, in the top (front) portion of the steel plate, the temperature significantly drops after hot rolling, and in addition, the super cooling is liable to occur because of turbulent flows of the cooling water, consequently causing camber of the steel plate. Especially, when the induced laminar flow nozzles as described in Japanese Examined Patent Publication No. 63-4604 are used, since the cooling water once returned into the water tank after cooling of the steel plate is used to cool the center portion of the steel plate, the temperature of the water is high. This causes significant super cooling of the top portion of the steel plate, thereby further facilitating occurrence of camber.
Japanese Examined Patent Publication No. 5-61005 discloses a method proposed to prevent such super cooling in the top portion of the steel plate. According to the method, a shield plate movable downwardly of the steel plate is installed, and cooling water drawn up from the lower surface side is thereby prevented from going up to the upper surface of the steel plate.
According to the method, however, the top portion of the steel plate is not cooled at any time because of the shield plate, so that uniform cooling cannot be performed in the longitudinal direction of the steel plate.